ATLANTA — Could this traffic-clogged Southern city, long derided as the epitome of suburban sprawl, really be discovering its walkable, bike-friendly, density-embracing, streetcar-riding, human-scale soul?

The answer is evident in the outpouring of affection that residents here have showered on the Atlanta BeltLine, which aims to convert 22 miles of mostly disused railway beds circling the city’s urban core into a biking and pedestrian loop, a new streetcar line, and a staggeringly ambitious engine of urban revitalization.

Even though just a small fraction of the loop trail has been completed, Atlantans, in one of the purer expressions of America’s newly rekindled romance with city life, have already passionately embraced the project. And like any budding romance, it is full of high hopes — for an Atlanta that is more racially integrated, less congested and, in a change refreshing to many here, more focused on improving the lives of residents rather than just projecting a glittering New South image to the rest of the world.

It’s not just Atlantans who see something that is potentially transformative.

“It’s the most important rail-transit project that’s been proposed in the country, possibly in the world,” said Christopher B. Leinberger of the George Washington University School of Business, who follows urban redesign projects and has for years called Atlanta “the poster child of sprawl.”

More than 30,000 people have taken a three-hour bus tour of the proposed loop; the answer to “Have you taken the tour?” has become a kind of litmus test of Atlanta civic pride.

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“It was just an idea, really,” Ryan Gravel, who first submitted the BeltLine idea to city officials in 2001, said recently. “I never imagined we’d actually do it.”Credit
Dustin Chambers for The New York Times

Last year, more than 1.3 million people used a completed two-mile path along the loop, the Eastside Trail, which opened in 2012, and a second, three-mile section of the path is under construction on the city’s historically African-American west side. On Saturday, tens of thousands of residents strutted their way along the existing trail in the annual BeltLine Lantern Parade, begun in 2010, that borrows much from the culture of New Orleans.

To hear the parade organizer, Chantelle Rytter, describe it, the Atlanta pageant might as well be a jazz funeral for the death of the city’s old reputation, which she sums up in three words: “Soulless parking lot.”

She added: “There’s a different way to live now because of the BeltLine.”

Such enthusiasm for what is, for now, little more than a glorified sidewalk says much about the social trends that are reinvigorating urban America. The current decade has been one of population growth for many of the United States’ largest cities. But Atlanta previously experienced decades of population loss because of suburbanization and white flight.

The tide has turned significantly in recent years. Planners now say Atlanta’s population, which stands at about 463,000, could double in the next 15 years. Many of the new residents could end up living along the BeltLine.

In a study this year, Mr. Leinberger and a colleague, Michael Rodriguez, showed that areas they identified as “walkable urban places” in the nation’s 30 largest metro areas were gaining market share over car-dependent suburban areas for “perhaps the first time in 60 years,” and earning higher rental premiums.

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People played kickball at a park on a trail on the BeltLine’s south side.Credit
Dustin Chambers for The New York Times

The High Line in New York, which turned an elevated stretch of Manhattan rail line into a linear park, is perhaps the best known of the nation’s urban infrastructure makeovers. Chicago’s has also converted an old elevated track into a greenway, christening it the 606. Miami’s Underline is reimagining 10 miles of underused land under its elevated Metrorail system as an art-lined “urban trail.”

Still, many say Atlanta’s plans stand out.

Private investment along the entire proposed route has surged to $3 billion. Foundations and private donors have given more than $54 million for paths, parks and other amenities. Home prices have risen in formerly overlooked working-class neighborhoods where the BeltLine is set to expand.

Candidates in the 2017 mayoral race, meanwhile, are turning BeltLine promises into central elements of their campaigns.

“If you like the BeltLine now, you’re going to love it when I’m your mayor,” says the campaign website for Cathy Woolard, a former City Council president.

The BeltLine idea was submitted to city officials in 2001 by a former Georgia Tech graduate student, Ryan Gravel. He grew up in the Atlanta suburbs, but had spent a year studying in Paris, where he got around without a car.

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SEPT. 11, 2016

By The New York Times

On a weekday afternoon in late August, as a packed BeltLine tour bus made its way through both charming historic neighborhoods and blocks plagued by drugs and crime, a guide called Mr. Gravel a “rock star” of urban planning.

“It was just an idea, really,” said Mr. Gravel, now a planner in Atlanta. “I never imagined we’d actually do it.”

Mr. Gravel and other advocates maintain great expectations. Upon completion in 2030, they say, the $4.8 billion project will connect 45 neighborhoods — rich and poor, black and white — thus easing old divisions of class and race. Organizers say it will promote healthy living and reduce obesity, and will provide new jobs, affordable housing, performance space, areas for urban farming and public art, as well as 2,000 acres of new and upgraded parks.

For all its economic success, locals have long known that Atlanta has had numerous unmet needs.

Mark Pendergrast, an Atlanta-born author, in a forthcoming book about the BeltLine, notes that the city, by at least one measure, suffers from the worst income-inequality gaps of any major American city; soul-deadening sprawl and commuting times; and neighborhoods that have been chopped up by highway construction and mangled by misguided 20th-century “urban renewal” projects.

For Joe Peery, 54, a commercial artist and longtime Atlantan, the BeltLine feels like a shift in the way the city conceives its big dreams. In the past, he said, Atlanta disappointed him with its big projects. The 1996 Summer Olympics struck him as corporate and cheesy: “a huge money grab,” he said. In contrast, the BeltLine lavishes attention on the neighborhoods where — as Mr. Peery and Ms. Rytter, the Lantern Parade organizer, would both agree — Atlanta’s low-key soul resides.

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Ellen Dunham-Jones, a Georgia Tech professor, largely abandoned driving for biking two years ago, in part thanks to the BeltLine, her new path to the grocery store.Credit
Dustin Chambers for The New York Times

“If not for the development of the BeltLine, I would have been driven out of here,” Mr. Peery said.

Mr. Gravel is surprised that the existing BeltLine has become such a gathering spot — a place to promenade, take outdoor yoga classes, and wander in and out of trendy restaurants.

But he and others know there are challenges ahead. Much of the project’s future funding will hinge on whether voters will approve, in November, two citywide ballot measures that will raise sales taxes by a total of nine-tenths of a cent.

Gentrification fears are also widespread. The city has built only a small fraction of the 5,600 affordable housing units it promised along the loop, largely because the recession from 2007 to 2009 depressed property values and lowered the revenue from a tax-increment funding plan.

Officials at Atlanta BeltLine Inc., the quasi-governmental agency overseeing the project, have pointed to other plans they hope will keep low-income residents along the BeltLine. But some residents are skeptical in a city that has torn down nearly all of its traditional public housing complexes in recent years.

“Instead of helping poor people around here fix up their property, they’re going to give them pennies on the dollar and they’re going to move,” said Lena Shepard, 79, a shopper at a west side grocery store along the BeltLine.

But Shudarrian Butler, 30, a barber working nearby, was looking forward to the new path. Maybe more whites would come to this neighborhood, he said. And maybe that was a good thing.

“It may blur that racial line a little bit,” he said. “Maybe we’ll learn to live amongst each other.”

A version of this article appears in print on September 12, 2016, on Page A1 of the New York edition with the headline: Atlanta’s Path to Rebirth Starts With a Sidewalk. Order Reprints|Today's Paper|Subscribe